The cancer death rate has dropped again. Here's why.

The nation's overall cancer death rate declined 1.7 percent in 2015, the latest indication of steady, long-term progress against the disease, according to a new report by the American Cancer Society. Over nearly a quarter-century, the mortality rate has fallen 26 percent, resulting in almost 2.4 million fewer deaths than if peak rates had continued.

But the report, released Thursday, shows that Americans' No. 2 killer remains a formidable, sometimes implacable, foe. An estimated 609,000 people are expected to die of the ailment this year, while 1.74 million will be diagnosed with it.

Cancer Statistics 2018, the organization's annual look at incidence, mortality and survival, tracks the decades-long decline in mortality as driven largely by falling death rates among four malignancies — lung, breast, prostate and colorectal cancer. Ahmedin Jemal, the group's vice president for surveillance and health services research, said the decreases largely reflect reduced smoking and advances in prevention, early detection and treatment.

The 10 leading cancer types for estimated deaths in the United States in 2018. (American Cancer Society)

Overall, the cancer death rate has dropped from 215.1 per 100,000 population in 1991 to 158.6 per 100,000 in 2015. U.S. rates of cancer incidence over the past decade were stable for women and decreased by about 2 percent annually for men. Still, the lifetime probability of being diagnosed with the disease is slightly higher for men than for women, with adult height accounting for about a third of the difference. Studies have shown that taller people have a greater risk of cancer.

Trends in U.S. cancer incidence and mortality since 1975. (American Cancer Society)

The new report also found that the racial gap on cancer death rates is narrowing: The 2015 mortality rate was 14 percent higher in blacks than whites, compared with a peak of 33 percent in 1993. But that trend masks significant disparities among age groups. Among people 65 and older, the death rate for blacks was 7 percent higher than for whites, a smaller disparity that likely reflects the effects of Medicare's universal health-care access. Among Americans younger than 65, the mortality rate was almost a third higher among blacks than whites — with even larger disparities in many states.

For younger people, “disparities are huge,” Jemal said. “We have to improve access to prevention, detection and treatment.”

Cancer is the second leading cause of death, after heart disease, for both men and women in the nation as a whole. But it's the leading killer in many states and among Hispanic and Asian Americans, the report noted.

The death rate dropped 39 percent from 1989 to 2015 for female breast cancer and 52 percent from 1993 to 2015 for prostate cancer. It also fell 52 percent from 1970 to 2015 for colorectal cancer, the report said, although since the mid-2000s that rate has actually increased slightly in individuals younger than 55.

The data show that a swift increase of melanoma incidence appears to be slowing, especially among younger people, but that liver cancer is rising rapidly in women, partly due to hepatitis C infections among baby boomers, Jemal said.

Otis W. Brawley, chief medical officer of the American Cancer Society, said in a statement that the report underscores the importance of continued efforts to discourage tobacco use. While the reduction in cigarette smoking has pushed down mortality rates, “tobacco remains by far the leading cause of cancer deaths today, responsible for nearly 3 in 10 cancer deaths.”

Laurie McGinley covers health and medicine for The Washington Post. She focuses on the Food and Drug Administration as well as cancer research and treatment. She was previously The Post's health, science and environment editor.